The present invention relates in general to the field of polarization transformation of light and, more particularly, to polarization transformation of optical signals exhibiting polarization mode dispersion.
Single-mode optical fiber is used in a variety of telecommunications systems. Despite its name, single-mode optical fiber actually transmits light in two distinct polarization modes. In a perfectly symmetrical single-mode optical fiber, these two modes travel through the fiber in exactly the same manner and are otherwise indistinguishable. However, imperfections in the fiber, either created during manufacture or caused by some external force on the fiber, can cause the refractive index of the glass core to differ slightly for light in the two different polarization modes, an effect called birefringence.
Birefringence associated with an optical fiber will cause the light in the two different polarization modes to travel at differing speeds. The birefringence encountered can be both uniform (e.g., a uniform manufacturing defect) and random. If the light traveling down the fiber is a typical optical pulse train used for telecommunications, each pulse initially might have components in both polarization modes. After traveling a distance down the fiber the two polarization components of the pulses will be separated in time. This time separation is called differential group delay. The statistical accumulation of differential group delay due to random polarization shifts and distribution of birefringence in the optical fiber is known as polarization mode dispersion (PMD). If it is too great, PMD causes the pulses to spread out thereby making it difficult to resolve individual pulses and thus transmit data without introducing transmission errors.
Polarization mode dispersion occurs in an optical fiber as a result of a small residual birefringence that is introduced in the fiber core by asymmetric internal stress or strain as well as random polarization coupling due to external forces acting upon the fiber. Thus, polarization mode dispersion may severely impair the transmission of a signal in an optical fiber network. It is well known that polarization mode dispersion has different effects on certain polarization components of an optical signal propagating through an optical fiber transmission line, such that differential time delays occur among the components as they travel through the fiber. These differential time delays can range from about 0.1 ps/(km)1/2 for low-PMD optical fibers of recent manufacture to several ps/(km)1/2 for single-mode optical fibers of older manufacture. For long-distance optical fiber links, e.g., a 100 km terrestrial transmission system using single-mode fiber, the differential time delay that can result from polarization mode dispersion may be more than 20 ps. Large time delays occurring between different polarization components can cause significant broadening of the optical pulses propagating through an optical link. This is a particular problem in digital lightwave systems operating at bit periods comparable to PMD, e.g., at least 10 Gbps per transmitted-wavelength-channel.
If the birefringence causing polarization mode dispersion were stable over time, it would be relatively simple to correct for the problem. However, random or time-varying mechanical stress on the deployed fiber leads to unpredictable polarization mode dispersion. Similarly, dynamic environmental changes result in polarization mode dispersion changes that can last for variable periods of time and vary with wavelength. In addition to diurnal heating and cooling, even faster thermal and mechanical effects, such as vibration from passing vehicles, fiber movement in aerial spans, and cabling disturbances by workers can cause polarization mode dispersion that possesses even greater variability. The rapid variation of these effects (e.g., on the order of a fraction of a millisecond to tens of seconds) suggests the need for relatively rapid corrective systems to preserve the integrity and lower the error rate of the optical data transmission. Moreover, the unpredictable nature of the resulting polarizations suggests the need for corrective systems that can adapt to a wide range of changes in birefringence.
Most devices that are intended to mitigate the problems of PMD do so by applying an appropriate delay to the faster of the two polarization components that make up the PMD degraded optical pulses. To do so, these devices should continuously and rapidly transform the state of polarization of these two polarization components to a known state, thereby controlling the polarization states. Continuously adjustable or xe2x80x9cendlessxe2x80x9d polarization transformers provide continuous control of the polarization state for a wide range of input polarizations. The simplest example is a rotatable wave plate. Unfortunately, most devices of this nature have relatively slow response times (perhaps on the order of tens or hundreds of milliseconds), and so they are not the most desirable devices to use to correct for polarization mode dispersion. A variety of devices with faster response times are available, but these devices generally have a limited range through which they can transform a polarization state and require resetting once they have reached their limit. Reset cycles can give rise to periods of unacceptable loss in overall system performance. In addition, multiple limited-range devices need to be combined in series, each device having a polarization transformation range that covers a range different from, but possibly overlapping with, the other devices. Such stacks of devices can still suffer from problems associated with reset cycles, as well as increased complexity and signal loss.
Accordingly, it is desirable to have polarization transforming devices, and particularly polarization transforming devices for use in polarization mode dispersion compensators, that have adequate response time and solve or alleviate the other problems of prior art devices.
It has been discovered that a polarization transformer can be constructed using a continuously adjustable polarization transforming device and a limited-range adjustable polarization transforming device. In general, the response time of the limited-range adjustable polarization transforming device is faster than that of the continuously adjustable polarization transforming device. When the two devices are properly controlled using error signals derived from a transformed optical signal, the polarization state of the optical signal can be adjusted with sufficient speed and without the loss of control associated with reset cycles.
Accordingly, one aspect of the present invention provides a polarization transformer operable to reorient polarization components of an incident optical signal. The polarization transformer includes a continuously adjustable retarder and a limited-range adjustable retarder. The continuously adjustable retarder is operable to provide reset-free operation and continuous control of a polarization state of the optical signal. The limited-range adjustable retarder is located in optical communication with the continuously adjustable retarder and is operable to provide limited-range control of the polarization state of the optical signal.
Another aspect of the present invention provides a system for compensating for polarization mode dispersion in an optical signal. The system includes a polarization transformer, a delay system, and a controller. The polarization transformer is operable to reorient polarization components of an incident optical signal and includes a continuously adjustable retarder and a limited-range adjustable retarder. The continuously adjustable retarder is operable to provide reset-free operation and continuous control of a polarization state of the optical signal. The limited-range adjustable retarder is located in optical communication with the continuously adjustable retarder and is operable to provide limited-range control of the polarization state of the optical signal. The delay system is operable to adjust the relative delay between a first reoriented polarization component of the optical signal and a second reoriented polarization component of the optical signal. The controller is coupled to the polarization transformer and is operable to provide control signals to the limited-range adjustable retarder and the continuously adjustable retarder.
These and other aspects of the invention have numerous advantages. For example, the present invention provides a polarization controller having fewer devices that need to be controlled. This is particularly useful since each device may need to be controlled individually by phase sensitive detection, and thus additional frequencies would need to be reserved in the network for dithering. Also, by decreasing the number of devices used in polarization transformation, lower insertion losses are achieved.
The foregoing is a summary and thus contains, by necessity, simplifications, generalizations and omissions of detail; consequently, those skilled in the art will appreciate that the summary is illustrative only and is not intended to be in any way limiting. As will also be apparent to one of skill in the art, the operations disclosed herein may be implemented in a number of ways, and such changes and modifications may be made without departing from this invention and its broader aspects. Other aspects, inventive features, and advantages of the present invention, as defined solely by the claims, will become apparent in the non-limiting detailed description set forth below.